At present, tablets, especially tablets used in the pharmaceutical field, are made by compressing a powder or granular product under high pressure.
This operation is performed by machines known as rotary tablet presses that essentially comprise a main operating unit with a central hopper designed to enable the product to be volumetrically dosed into a plurality of holes or dies located in a first rotating disc and equally distributed round the circumference of the disc. The first disc is positioned between another two discs that rotate in synchrony with it. These two discs mount a corresponding plurality of upper and lower, reciprocating punches driven by appropriate cam mechanisms that move the punches towards one another in pairs in a vertical direction in such a way that they enter the corresponding hole or die and compress the product to form a tablet. The upper and lower punches are then moved away from each other and the tablet made is ejected into an appropriate container.
The operating unit described above is located inside a sealed chamber structure isolated from the outside environment so that the production zone is kept clean and free of contamination by external agents, thus ensuring compliance with the safety regulations and hygiene specifications of the pharmaceutical industry.
Each punch normally comprises a cylindrical shank equipped at one end with a working head having a rounded, cap shaped surface. The above mentioned cam means act on the rounded surface of the working head in such a manner as to drive each pair of punches up and down into a die located in the rotating disc.
In view of the high speeds reached by the punches during the compression of the product, each cylindrical shank must be protected not only against wear due to its rapid working movements but also against external agents that might prevent the shank from sliding smoothly against the sides of the die. To achieve this, each shank is usually partly covered by sealed protection means and supplied with a lubricating fluid. The protection means must therefore be designed to prevent contact between the lubricant and the product, as well as contact between the cylindrical shank and external agents, and must also provide the punches with an effective seal during the frequent cleaning and disinfecting cycles performed inside the production chamber.
In a first known solution described, for example, in U.S. Pat. No. 5,462,427, the sealed protection consists of a single plastic cover over the entire external surface of each rotating disc. For each punch, the cover has a concertinaed portion with an annular sector for each punch shank on each disc.
This type of protective cover is very effective but, because it is a single part and therefore has a complex shape, takes a considerable amount of time to be fitted over the discs and punches. This also greatly increases the cost of this type of solution.
To overcome these drawbacks, German patent DE 29516577 discloses a solution whereby each single punch, both upper and lower, that moves with vertical, reciprocating motion, is enclosed in a plastic concertinaed sleeve connected, at the far end relative to the working head, to an annular seat made in the disc, and at the near end to a groove made close to the punch head.
This type of sealed protection works well on reciprocating punches that move only straight up and down but cannot be used on punches which, in addition to the reciprocating vertical motion, also revolve about their respective, longitudinal axes as the turret rotates, the revolving motion being an effective technical solution now frequently adopted to enable the drive means to apply pressure uniformly on the rounded cap-shaped surfaces of the punch heads in order to reduce wear on the punch heads themselves.
In the solution taught by the above mentioned German patent DE 29516577, the impossibility of using punches that revolve about their longitudinal axes is due to the fact that the far end connection of the concertinaed sleeve to the mounting disc is rigid and, if the punch itself were also made to revolve, would cause the concertinaed sleeve to twist and break.